The problem of developing cardiac valve prostheses, capable of replacing in a satisfactory manner the natural valves of the human heart to correct their dysfunction due to congenital or acquired pathology, has a more than thirty-year history.
Over these decades, a vast number of valve prosthesis designs have been developed and used in practice. Generally, such a prosthesis comprises a return valve permitting forward blood flow in the open position of the closure member and preventing the reverse blood flow in the closed position of the closure member.
Owing to their superb hemodynamic characteristics ensuring a central and virtually laminar flow, the cardiological science has accepted for wide-scale application cardiac valve prostheses, in which the closure member is designed in the form of two semicircular cusps pivoted to the prosthesis body.
Known in the art is a cardiac valve prosthesis (ref. U.S. Pat. No. 4,689,046) which comprises an annular body, the inner surface of which defines a central opening for the passage of the forward blood flow. The annular body houses a closure member in the form of two cusps which pivot from the open position and back. A cusp has an ascending surface at the side of the ascending flow and a descending surface at the side of the descending flow, and also a lateral surface contacting the inner surface of the annular body in the closed position. The annular body is provided on the inner surface thereof with spaced recesses engageable by the projections of the cusps provided on the opposite sides of the lateral surfaces.
During operation of the aforesaid cardiac valve prosthesis, the cusps of the closure member do not alter their position relative to the body axis depending on a particular cardiac cycle moment, i.e., either in the closed position, or in the open position, or in the intermediate position. This situation causes the appearance of blood congestion zones which stimulate thrombus forming processes that lead to either a thrombosis of the prosthetic valve or thromboembolic complications. Furthermore, the local wear of individual portions of the annular body and cusps of the closure member, which cooperate constantly or cyclically with one another, reduces the service life of the cardiac valve prosthesis.
An attempt to overcome the above-mentioned drawbacks was made in a cardiac valve prosthesis described, for example, in U.S. Pat. No. 4,274,437. This prosthesis comprises an annular body having an inner surface defining an opening for the forward blood flow along the body axis and two valve members complementing each other and secured inside the body to prevent the backflow of blood in the closed position. The annular body is provided with a slot extending along the entire circumferential length of its inner surface and receiving projections provided on the opposite sides of each valve member to allow the valve members to turn from the closed to the open position and back. Each of the valve members has an outer convex surface and an ascending portion having a semicircular outer edge. The edge contacts the inner surface of the annular body and checks the backflow of blood in the closed position of the prosthesis. In addition to the turning movement from the closed to the open position and back, the valve members of this prosthesis can partially rotate about the axis of the annular body, thereby preventing the development of congestion zones and reducing the wear of the cooperating components of the prosthesis.
One of the drawbacks of this cardiac valve prosthesis is that it lacks means for producing the rotation about the body axis. The rotation can be generated in this prosthesis only by the fluctuating swirling in the blood flowing through the valve prosthesis. Obviously, the rotation of the valve members due to random flow irregularities would be chaotic and impossible to control in order to produce a positive effect. The relatively high profile of the cardiac valve further reduces the positive effect, because the cardiac structures surrounding the prosthesis inhibit the rotation of the valve members.
In the inventors' view, the most successful design is the cardiac valve prosthesis disclosed in European Application No. PCT/SU 88/00258. The cardiac valve prosthesis comprises an annular body having an inner surface that defines an opening for the forward blood flow, and a closure member received in the annular body and designed in the form of two cusps having an ascending and a descending surfaces and a lateral surface confined therebetween and contacting the inner surface of the annular body in the closed position to check the backflow of blood.
The annular body has a projection extending along the entire circumference of the inner surface thereof to engage slots provided on the opposite sides of the lateral surface of each cusp. Each cusp is provided with a detent to maintain a desired spacing between the cusps in the open position. The slot provided on each of the opposite sides of the lateral surface of the cusp is an open-ended groove. The profile of the lateral sides of the slot is a broken line having a first and a second portions. The like portions of the broken lines of the lateral sides of the slots on the opposite sides of the lateral surface of the cusp have different lengths.
Following below is a detailed description of the closing moment of one of the cusps of the aforesaid cardiac valve prosthesis. The excess pressure of the backflow of blood acting upon a cusp causes the latter to turn into a closed position as it bears along the intersection line of the broken lateral side of the slot against the surface of the projection provided on the inner surface of the annular body. In this case, since the like portions of the broken lines have different lengths the opposite sides of the lateral surface of the cusp move to a different distance relative to the inner surface of the annular body. This causes the cusp to turn about the axis of the annular body.
It follows from the above description of the operation of the cardiac valve prosthesis that the mechanism causing rotation of the cusps about the body axis is based on the difference in the forces of friction resulting from the interaction between the lateral side surface of the cusp slot and the projection on the inner surface of the annular body. To allow long-term faultless operation, anti-friction materials, for example, pyrolytic carbon, are used as the material of the prosthesis components. As a result, the aforesaid friction forces are very small. Therefore, the rotation of the cusps about the body axis would be greatly influenced by various fluctuation processes occurring in the forward and reverse blood flows through the prosthesis. These flows produce chaotic movement, thereby minimizing the effect of the cardiac valve prosthesis.
Besides, the small thickness of the cusp makes for a small difference in the length of the broken line portions of the slots on the opposite sides of the cusps, thereby obviating the possibility of the cusps rotating at a sufficiently high speed about the body axis which, in turn, prevents blood flow swirling essential for normal operation of the heart.